Process for preparation of high-strength alloy of titanium and ferritic structure

ABSTRACT

Low-alloy high-strength titanium steel and a process for preparing said steel by solution heat treating and thereafter controlling the cooling rate to the precipitation-hardening temperature range required to produce a steel characterized by a yield strength in the range of 60,000 to 120,000 p.s.i.

United States Patent 72] Inventors [54] PROCESS FOR PREPARATION OF HIGH- STRENGTH ALLOY OF TITANIUM AND FERRITIC STRUCTURE 10 Claims, No Drawings [52] 11.8. C1 148/134, 148/12, l48/l2.3, 148/142 [51] 1nt.C1 C21d7/14, C2 ld 9/46 [50] Field 01 Search 148/36, 124,142,143,12,12.3,134;75/123 M [56] References Cited UNITED STATES PATENTS 2,624,687 1/1953 McMullan 148/36 X 2,770,563 11/1956 Herzog 148/36 2,806,782 9/1957 Faber 75/58 X 2,853,379 9/1958 Althouse 148/36 X 2,858,206 10/1958 Boyce 75/126 3,010,822 11/1961 Altenburger. 75/123 3,102,831 9/1963 Tisdale 148/12 3,333,987 8/1967 Schrader 148/2 FOREIGN PATENTS 939,616 7/1961 Great Britain 75/123 OTHER REFERENCES Precipitation in Mild Steels Containing Small Additions of Niobium," Gray eta1.,.1ouranl of The iron and Steel Institute, Aug. 1965, pages 813- 818 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-John Stelmah ABSTRACT: Low-alloy high-strength titanium steel and a process for preparing said steel by solution heat treating and thereafter controlling the cooling rate to the precipitationhardening temperature range required to produce a steel characterized by a yield strength in the range of 60,000 to 120,000 p.s.i.

PROCESS FOR PREPARATION OF HIGH-STRENGTH ALLOY OF TITANIUM AND FERRITIC STRUCTURE This invention relates to the production of a new series of precipitation-hardened ferritic steels which utilize titanium as the major strengthening agent. These steels, when produced in accordance with the specified processing cycle, develop yield strengths in the range of 60,000-120,000 p.s.i. and exhibit desirable cold formability and weldability characteristics. The described process is particularly adaptable to the production of flat-rolled products and can be utilized in the production of other steel products.

The present invention provides for an alloy of ferritic structure having a yield strength in the range of 60,000 to 120,000 p.s.i. consisting essentially of, by weight, 0.03 to 0.20 percent carbon, 0.04 to 0.35 percent titanium, and the principal portion of the remainder being iron with ordinary impurities. It further provides an alloy as described above wherein the remainder includes manganese in the range of 0.3 to 1.5 percent of the total alloy weight. The remainder also includes the residue of a killing agent (or deoxidizing agent) selected from the group consisting of aluminum, silicon, additional titanium and zirconium. Preferably, the killing agent is aluminum comprising 0.02 to 0.07 percent of the total composition from which said alloy is prepared. 1n its preferred fonn, the alloy consists essentially of 0.04 to 0.10 percent carbon, 0.02 to 0.32 percent titanium, and the balance iron with residual impurities in ordinary amounts and exhibits a yield strength of 60,000 to 1 10,000 p.s.i.

The invention further provides for a process for producing an alloy of fern'tic structure having a yield strength in the range of 60,000 to 120,000 p.s.i. comprising the steps of (A) solution heat-treating an alloy composition consisting essentially of 0.03 to 0.20 percent carbon, 0.04 to 0.35 percent titanium, and the principal portion of the remainder being iron with residual impurities in ordinary amounts; (B) quenching said alloy composition to a temperature in the range of l,000 to l,225 F.; and (C) precipitation treating said alloy composition. It is further provided that the alloy composition is cooled to room temperature after the step of precipitation treating.

in its preferred form the process provides for solution heattreating which comprises heating the alloy composition to a uniform temperature above about 2,000 F. for at least 5 minutes. The process further provides for quenching which comprises cooling the alloy to a temperature of not less than l,500 F. at a rate greater than 1 P per second and thereafter maintaining a cooling rate in the range of 7.5 to 75 F. per second, preferably at a rate greater than F per second until reaching a temperature in the range of l,000 to l,225 F. by directing a coolant such as water upon one or more surfaces of the alloy. Preferably, the precipitation treating comprises cooling at a rate generally less than 100 F. per minute until reaching a temperature of about 900 F.

The present invention additionally provides for steel produced from the composition described above according to the process described above and being characterized by yield strengths in the range of 60,000 to 120,000 p.s.i.

The influence of chemical composition and processing on the mechanical properties of precipitation hardened steels produced by the process of the invention will now be described in detail.

The following example discloses the ranges of the principal elements of a composition from which a low-alloy steel within the scope of the present invention is prepared:

Element Percent by weight Carbon (C) 0.030.20 Manganese (Mn) up to 1.5

Sulfur (S) 003 maximum Phosphorus (P) 0.015 maximum Silicon (Si) up to 0.30 Nitrogen (N) up to 0.01 Titanium (Ti) 0.04-0.35 Aluminum (Al) 0.02-0.07

(or other killing agent) Iron constitutes the base of the alloy and comprises the balance of the composition with the exception of insignificant amounts of impurities incident to usual steelmaking practice such as iron oxides, other metallic oxides, and the like.

An important element of the steels produced in accordance with the invention is titanium (Ti). Ti is present in the range of 0.04-0.35 percent depending on the strength level desired. A linear relationship between yield strength and Ti content has been observed up to 0.3 percent Ti. It is believed that the principal strengthening influence upon "precipitation-hardened steels produced by the process of the invention is a fine dispersion of titanium carbonitride (TiCN) formed during transformation to ferrite. The ferrite matrix produced is almost completely void of pearlite.

The carbon and nitrogen concentration influence the strength of these steels as shown in table 1 below for flat-rolled products at two titanium concentrations produced within the processing conditions of the invention. Specifically, the steels tabulated in table 1 were forged and hot rolled from 50-pound air-induction melted laboratory ingots. Samples were prepared from forged 'Va-inch plates. The samples were processed in a laboratory apparatus which simulates an actual hot strip mill installation and the conditions connected therewith. The processing steps comprised heating the samples at a solution temperature of 2,100 F. for 8 minutes, air cooling during hot deformation at a rate of 3 F. per second, finishing hot deformation at a temperature greater than 1,500 F., quenching by fluid spray such as water, to a temperature in the range of 1,000-l ,300 F., followed by a cooling sequence which provides cooling at a rate generally less than F. per minute until reaching a temperature of about 900 F. and thereafter air cooled' to room temperature. Standard procedures were used to test the tensile and impact specimens obtained from the samples processed by the foregoing procedure.

As shown in table 1, at the 017/019 titanium concentration, an increase in carbon content from 0.06-0.16 percent at the 0.002 nitrogen level resulted in a decrease of 30,000 p.s.i. in the yield strength. Raising the nitrogen level from 0.0017 to 0.01 percent resulted in a similar loss in strength. The same trends are shown, but to a lesser magnitude on the steels containing 0.04 percent titanium. For optimum strength in these steels, the carbon concentration should be between 0.04 and 0.10 percent. Higher carbon concentrations could be utilized but at a sacrifice in strength. The lowest nitrogen level is preferred and should be below 0.01 percent to produce flatrolled products with 90,000 p.s.i. minimum yield strength.

The steels of the present invention exhibit desirable welding characteristics because the normal elements which cause difficulty in welding are not present in significant amounts; that is, the manganese and carbon concentrations are kept low enough to achieve good weldability. Higher manganese levels can be used to produce higher strengths with some sacrifice in weldability.

Nickel and copper may be added to these steels without detriment to the mechanical properties and will improve the atmospheric corrosion resistance as is known to those skilled in the art. However, nickel and copper are not essential to the primary strengthening mechanisms in the titanium steels.

The addition of other strengthening agents such as vanadi- TABLE 111 um and molybdenum will result in strength increases above those associated with the particular titanium base. The addition of vanadium would be most beneficial where a high aj r Co i g Yield Ten-ll: nitrogen residual level occurs, since the presence of vanadium 5 3 g? 33; 1 g; g 'l results in the formation of vanadium nitride which increases c c the strength and also decreases the amount of nitrogen that is available for association with the titanium. 3 Q 1: 33m 31m A unique method has been discovered for processing the 0.23 103300 Moo steels described herein to impart strength and other physical l 0.23 Ti 7% 89,000 109,100 properties to such steels on present-day strip mill facilities 013 E 2:32: 22:38 which steels heretofore could only be developed by the use of 3:8: 75300 expensive techniques and expensive alloy ingredients. The 004 70.300 first step comprises subjecting the metal to a high solution 0.04 Ti 1% 50,350 67,100 temperature. A highsolubility of Ti in the austenite is necessal5 Ti 4 3 63Mo ry so that the reprecipitation of the Ti as TiCN can be controlled to insure a fine dispersion throughout the ferrite matrix. In the case of the Ti steels, the solution temperature d Id strength must be above 2,000 F. which corresponds to the normal ecreasem yle temperature attained in the conventional hot strip slab reheat 2O f development of max'mum f F z i a a fumlaples. 'Ihe sz eel ils subjected to the soLutionJgapegaturg 3:32:51: g'gzgzri fifsge im c; unti eate uni orm y to a temperature a ove an then held at thattemperature for at least 5 minutes to insure f d fiih t'hi g g f t t! PT (P :l: th Q that the TiCN goes fully into solution, colncl es wl ans orma ion 0 cm e ra er an prlo in commercial steelmaking practice, the steel ordinarily P P auslenlle) It f fi t would be subjected to hot deformation after solution heat l l p tf l g as i ggg z gP tF;;'- ;;"g g:? treatment. However, no deformation is required to develop 1 6 range 0 1 i ee 13 e 18 r g the desirable properties in the t l f th present i i establishes the desirable temperature range in which the steel Whether or not hot deform ti i f d, a li is coiled. Our studies indicate that if a uniform coiling temrate in excess of 1 F. per second is maintained while cooling perature is Obtained, Variation in Strength Should ur from the solution heat-treating temperature. This cooling rate from headto tail or from center to edge, widthwise, in a coil assures that transformation of austenite to ferrite will not comprocessed In a conventional strip mill. mence above l,500 F. As discussed below, a rapid cooling These titanium steels exhibit elongations greater than 20 rate through the austenite transformation range is desired to percent in 2 inches and a reduction in area greater than 50 achieve tit; hiilgghyield stlrengthsirli1thegtlfels oipaeserliltgpZrcentt in oth sarip andt plptg. [A liplnefigilal cold grrgalpligly vention. e 1 er coo lng rate 5 ou e esta is e we c arac ens 10 IS emons ra e y e a l l y o n a fore the temperature at which transformation commences is strength levels through 180 without cracking to an inside reached. For this reason, any hot deformation which is perdiameter equal to the material thickness. The impact strength formed should be completed (finished) at a temperature in the 60,000/70,000 p.s.i. yield strength range is greater than above l,500 F. The precise temperature at which austenite l5 foot-pounds at atest temperature of -50 F. tranlsfgrmation begins in a particeular alloy cgnllpodsiltion being fWlhile we htiiiVe desgn'kped alpretseilt tpreferred einbodtimens coo e at a particu ar rate can ascertaine y l atometric o t e inven on an ave 1 us ra e a presen pre erre methods as described in the Metals Handbook, 1948 Edition, method of practicing the same, it is to be distinctly understood pages 168-174 or by any other convenient method. that the invention is not limited thereto but may be otherwise The effect of maintaining a cooling rate greater than 1 F. variously embodied and practiced within the scope of the folper second during cooling from the solution heat-treating temlowing claims. perature is illustrated in table lI-A below for a 0.18 percent "Ii What is claimed is: I glee]. The data in tablehll wczlas deygelgpfed by Iprofessing stliels in h l. A proctltgs {or prtoductilr'lig a kllledfagtyfigienpgg (s)t0rgctur e e same manner as t at escri e or ta e a ove. n mavlngayie s reng in erangeo o p.s.|. crease in the cooling rate from l F. to 3 F. per second and having a bendability characteristic demonstrable by a resulted in yield strength increase of approximately 10,000 capability of being bent through an arc without cracking to an p.s.i. inside diameter equal to the thickness of the product, compris- TABLE 11 Parameters Yield Tensile Processing variable investi- Major alloying strength strength evaluated gated elements (p.s.l.) (p.s.l.)

A. Cooling rate between solution temperature and {3 F/second .18 weight percent TL. 97, 600 116, 950 finish deformation tem- 1 F/seeond .do 87, 300 106, 200 perature.

- H88: .14 velgl'lli percent TL. 3,3165% 32, gag B. Temperature after 0001- c o ing at F/mnd- 532 F .11.... 31%;???) 33; i3

1,250 F do 70,400 ,1

The effect of cooling rate from the finish deformation teming the steps of: perature to the precipitation treating temperature on the yield strength is shown in table 111, the data for which having been developed in the same manner as described for table I above:

A cooling rate of 45 F. per second yielded the highest strength at both high and low titanium levels. A decreased in cooling rate from 45 F. per second resulted in a marked A. solution heat-treating an alloy composition essentially of 0.03 to 0.20 percent carbon, 0.04 to 0.35 percent titanium, and the principal portion of the remainder being iron with residual impurities in ordinary amounts and which are not subversive of the above yield strength characteristic;

B. quenching said alloy composition to a temperature in the range of 1,000 to 1,225 F.; and C. further cooling said alloy composition at a rate less than 100 F. per minute until reaching a temperature of about 900 F.

2. A process as described in claim 1, wherein:

said remainder includes manganese in the range of 0.3 to

1.5 percent of the total alloy weight.

3. A process as described in claim 1, wherein:

said remainder includes the residue of a killing agent selected from the group consisting of aluminum, silicon, additional titanium, and zirconium.

4. A process as described in claim 3, wherein:

the killing agent is aluminum comprising 0.02 to 0.07 percent of the total composition from which said alloy is prepared.

5. A process as recited in claim 1, in which the solution heat-treating comprises heating the alloy composition to a uniform temperature above about 2,000 F. for at least 5 minutes.

6. A process as recited in claim 1, in which the alloy composition is cooled to room temperature after said step of further cooling.

7. A process as recited in claim 1, in which said quenching comprises cooling the alloy to a temperature of not less than l,500 F. and thereafter maintaining a cooling rate in the range of 7.5 to 75 F. per second until reaching a temperature in the range of l,000 to l,225 F.

8. A process as recited in claim 7, in which said cooling rate in the range of 7.5 to 75 F. per second is maintained by directing a coolant upon one or more surfaces of the alloy.

9. A process for producing an alloy of ferritic structure having a yield strength in the range of 60,000 to 110,000 p.s.i. comprising the steps of:

A. solution heat-treating an alloy composition consisting essentially of: 0.04 to 0. l0 percent carbon, 0.20 to 0.32 percent titanium and balance iron with residual impurities in ordinary amounts B. quenching said alloy composition to a temperature in the range of 1,000 to l,225 F.; C. Further cooling said alloy composition at a rate less than F. per minute until reaching a temperature about 900 F.; and thereafter D. cooling said alloy composition to room temperature.

10. A process for producing an alloy of ferritic structure having a bendability characteristic demonstrable by a cap'ability of being bent through an are without cracking to an inside diameter equal to the thickness of the product, comprising the steps of:

A. solution heat-treating an alloy composition, which composition comprises 0.03 to 0.20 percent carbon, 0.04 to 0.35 titanium, and the principal portion of the remainder being iron with ordinary impurities, at a uniform temperature of at least 2,000 F. for at least 5 minutes.

B. cooling said alloy composition to about l,500 F. at a rate greater than l F. per second; and

C. further cooling said alloy composition to a temperature in the range of l,000 to l,225 F. at a rate greater than 15 F. per second; and

D. still further cooling said alloy composition to about 900 F. at a rate generally less than l00 F. per minute.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION atent No- 3, 5,780 Dated December 7, 1971 Inventor(s) Richard A. Bosch, et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet [73] "The" should not appear as part of the assignee's name: column 1. line 26 "0.02" should read 0.20 line :7 "75F" should read 75 F column 2, line 26 "3E" should read 3 F line 30, "lOO F" should read 100 F TABLE I, under Carbon Content 1.10" should read 0.10 ---3 column 3, lines 30, 45, 50 '1 F" should read 1 F line 50, "3 F" should read 3 F --5 lines 7 4, Z6, F" should read 45 F TABLE II, "3 F/second", "l F/second" and "45 F/second" should read 3 F /second 1 F /second and 45 F /second respectively,- TABLE III, F/Sec. should read F /Sec. column 4, line 21, F" should read 75 F claim 1, Sub-para ragh C, F" should read 100 F claim 7, line 4 5 F" should read 75 F claim 8, line 2, "75 F" should read 75 F claim 9, Subgaragraph C should be separated from Sub-paragraph B, "100 F" should read 100 F claim 10, Sub-paragraph B, "1 F" should read 1 F --3 Sub-paragraph C, "15 F" should read g5 F Sub-paragraph D, "100 F" should read 100 F Signed and sealed this 12th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents M USCOMM-DC scan-Pas 75, GOVERNMENT 'IINTING OFFICE ll. 0'3-314. 

2. A process as described in claim 1, wherein: said remainder includes manganese in the range of 0.3 to 1.5 percent of the total alloy weight.
 3. A process as described in claim 1, wherein: said remainder includes the residue of a killing agent selected from the group consisting of aluminum, silicon, additional titanium, and zirconium.
 4. A process as described in claim 3, wherein: the killing agent is aluminum comprising 0.02 to 0.07 percent of the total composition from which said alloy is prepared.
 5. A process as recited in claim 1, in which the solution heat-treating comprises heating the alloy composition to a uniform temperature above about 2,000* F. for at least 5 minutes.
 6. A process as recited in claim 1, in which the alloy composition is cooled to room temperature after said step of further cooling.
 7. A process as recited in claim 1, in which said quenching comprises cooling the alloy to a temperature of not less than 1, 500* F. and thereafter maintaining a cooling rate in the range of 7.5 to 75* F. per second until reaching a temperature in the range of 1,000* to 1,225* F.
 8. A process as recited in claim 7, in which said cooling rate in the range of 7.5 to 75* F. per second is maintained by directing a coolant upon one or more surfaces of the alloy.
 9. A process for producing an alloy of ferritic structure having a yield strength in the range of 60,000 to 110,000 p.s.i. comprising the steps of: A. solution heat-treating an alloy composition consisting essentially of: 0.04 to 0.10 percent carbon, 0.20 to 0.32 percent titanium and balance iron with residual impurities in ordinary amounts B. quenching said alloy composition to a temperature in the range of 1,000* to 1,225* F.; C. Further cooling said alloy composition at a rate less than 100* F. per minute until reaching a temperature about 900* F.; and thereafter D. cooling said alloy composition to room temperature.
 10. A process for producing an alloy of ferritic structure having a bendability characteristic demonstrable by a capability of being bent through an arc without cracking to an inside diameter equal to the thickness of the product, comprising the steps of: A. solution heat-treating an alloy composition, which composition comprises 0.03 to 0.20 percent carbon, 0.04 to 0.35 titanium, and the principal portion of the remainder being iron with ordinary impurities, at a uniform temperature of At least 2,000* F. for at least 5 minutes. B. cooling said alloy composition to about 1,500* F. at a rate greater than 1* F. per second; and C. further cooling said alloy composition to a temperature in the range of 1,000* to 1,225* F. at a rate greater than 15* F. per second; and D. still further cooling said alloy composition to about 900* F. at a rate generally less than 100* F. per minute. 